1. Field of the Invention
The present invention relates to a method of manufacturing an electric resistance element.
2. Description of the Prior Art
Certain electric resistance elements have been manufactured by coating a resistance paste material prepared of carbon particles and organic synthetic resin, on the surface of an insulator by a method such as screen printing to form a resistance film.
A conventional method of manufacturing an electric resistance element of this type, comprises a step of kneading carbon particles and a solution of thermosetting resin to prepare a paste material; a step of screen-printing the paste material on the surface of a substrate (particularly an insulator such as a laminated plate of phenol resin or epoxy resin, an alumina ceramic plate or a metallic core substrate ) in the form of a pattern; a step of removing a solvent by application of heat and further heating the substrate at a temperature of, e.g., 100.degree. to 150.degree. C. to cure the resin thereby to form a resistance film; and a step of mounting an electrode on the part of the resistance film by a conductive paint such as silver paste. The thermosetting resin is prepared of phenol resin, epoxy resin, melamine resin, urea-formaldehyde resin, alkyd resin or acrylic resin, or a mixture thereof.
However, the electric resistance element manufactured by the conventional method has significant disadvantages. One such disadvantage is seen in that the resistance is extremely varied and inferior in stability relative to environmental conditions The most likely cause for such disadvantages is that the carbon particles are non-uniformly arranged in the resin while the resin does not adhere uniformly to the surfaces of the carbon particles whereby the interfaces between the carbon particles are varied, leading to instable contact resistances. This is based on the principle that the resistance of an electric resistance element is not determined by a volume specific resistance of carbon particles, but rather by contact resistances thereof.